Insulin's ability to stimulate chronic or growth responses was investigated by testing various insulin analogues's ability to stimulate 3(H) thymidine incorporation into normal human fibroblasts in culture. Using human insulin synthesized from amino acids, we have demonstrated that insulin molecule itself has the ability to stimulate growth and this ability is not due to any contamination of the preparation. In addition insulin's ability to stimulate growth appears to be much more conserved than its metabolic effects. In addition we have found evidence that in rat adipocytes, there could be interaction between insulin receptor and MSA (a growth factor) receptors.